From the web site:
Vintage Racing League: History of the AC Cobra etc
" AC Shelby Cobra,like many British specialist manufacturers, AC Cars had been using the smooth, refined Bristol straight-6 engine in its small-volume production, including its AC Ace 2-seater roadster. The engine was a pre-World War II design of BMW which by the 60s the company knew was considered dated. Bristol decided in 1961 to cease production of its engine and instead to use Chrysler 313cid (5.1 L) V8 engines. Although untrue, it is commonly believed that AC was left without a future source of power and that American ex-racing driver Carroll Shelby saved the company from bankruptcy. AC started using the 2.6 litre Ford Zephyr in all of its cars. In September 1961, Shelby airmailed AC a letter asking them if they would build him a car modified to accept a V8 engine. Shelby had previous experience with Anglo-American hybrids, having raced an Allard. He first went to Chevrolet to see if they would provide him with engines, but not wanting to add competition to the Corvette they said no. Ford however, wanted a car that could compete with the Corvette and they happened to have a brand new thin wall small block engine which could be used in this endeavor. "
Complete Article:
Cobra Owners Group - The Vintage Racing League